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FOUNDED  BY  JOHN  D.  ROCKEFELLER 


THE  EFFECT  OF  TEMPERATURE  ON 
THE  IONIZATION  OF  A  GAS 


A  DISSERTATION 

SUBMITTED    TO    THE    FACULTY    OF    THE    OGDEN    GRADUATE    SCHOOL 

OF    SCIENCE    IN    CANDIDACY    FOR    THE    DEGREE 

OF  DOCTOR  OF  PHILOSOPHY 

(DEPARTMENT  OF  PHYSICS) 


BY 

J.  HARRY  CLO 


IQII 


ITbe  THnfx>ersit£  of  Cbtcaoo 


THE  EFFECT  OF  TEMPERATURE  ON 
THE  IONIZATION  OF  A  GAS 


A  DISSERTATION 

SUBMITTED    TO    THE    FACULTY    OF    THE    OGDEN    GRADUATE    SCHOOL 

OF    SCIENCE    IN    CANDIDACY    FOR    THE    DEGREE 

OF  DOCTOR  OF  PHILOSOPHY 


(DEPARTMENT  OF  PHYSICS) 


BY 

J.  HARRY  CLO 


IQII 


THE  EFFECT  OF  TEMPERATURE  ON  THE  IONIZATION 

OF  A  GAS 

BY  J.  HARRY  CLO 

The  ultimate  purpose  of  this  experiment  was  to  determine 
whether  it  is  possible  to  change  the  kinetic  energy  of  the  molecule 
sufficiently  to  affect  the  stability  of  the  atom.  The  ionization  of 
gases  was  chosen  as  a  type  of  the  phenomena  which  involve  the 
separation  of  the  electron  from  the  atom  and  which  therefore 
depend  on  its  stability. 

The  experiments  made  heretofore  upon  these  phenomena  may 
be  divided  into  three  classes,  namely,  those  on  the  ionization  of 
gases,  those  on  the  photo-electric  effect  of  ultra-violet  light,  and 
those  on  the  emission  of  electrons  from  radioactive  substances. 

H.  L.  Bronson1  has  shown  that  upon  heating  radium  salts  under 
conditions  which  eliminate  radioactive  transformations,  volatiliza- 
tion of  products,  etc.,  the  ionization  of  a  gas  by  gamma  rays  is 
independent  of  the  temperature  of  the  radium.  But  the  intensity 
of  the  gamma  rays,  and  therefore  their  ionizing  power,  is  generally 
believed  to  depend  on  the  number  of  electrons  given  off  per  second 
from  the  radium.  Hence  it  would  seem  to  follow  that  the  rate  of 
emission  of  the  electrons  is  not  affected  by  the  temperature.  His 
observations  extended  from  — 180°  C.  to  1600°  C.  and  showed 
practically  no  variation  greater  than  i  per  cent. 

The  expulsion  of  negative  electrons  from  metals  under  the  in- 
fluence of  ultra-violet  light  was  shown  to  be  independent  of  the  tem- 
perature of  the  metal  by  Millikan  and  Winchester,2  Ladenburg3 
and  others.  Millikan  and  Winchester  made  observations  on 
different  metals  up  to  temperatures  about  350°  C.  Ladenburg 
experimented  on  platinum,  gold,  and  iridium,  varying  the  tempera- 
tures from  20°  C.  to  as  high  as  860°  C.  With  platinum  his  results 

1  Proc.  Roy.  Soc.,  A,  78,  494,  1907. 

2  Phil.  Mag.  (6),  14,  188,  1907. 

3  Verh.  der  Deutschen  Phys.  GeselL,  9,  165,  1907. 


n6  /.  HARRY  CLO 

show  no  variation  greater  than  about  5 . 5  per  cent  from  the  mean. 
For  gold  and  iridium  his  results  are  even  better. 

The  effect  of  temperature  on  the  ionization  of  a  gas  has  been 
investigated  by  J.  Perrin,1  McClung,2.  A.  Gallarotti,3  Herweg,4 
and  Crowther.5  With  the  exception  of  Perrin  none  of  these  ex- 
perimenters found  any  systematic  variation  of  the  ionization. 
Perrin  measured  the  ionization  produced  in  air  by  Roentgen  rays. 
Correcting  for  the  variation  of  the  ionization  with  density,  he  found 
it  to  be  proportional  to  the  absolute  temperature.  McClung  in- 
vestigated the  phenomenon  very  thoroughly,  using  Roentgen  rays 
as  an  ionizing  agent.  He  measured  the  ionization  in  air,  hydrogen, 
and  carbon  dioxide,  working  both  at  constant  pressure  and  at  con- 
stant density.  His  method  allowed  him  to  correct  for  the  varia- 
tion in  the  ionizing  power  of  the  rays.  For  air  at  constant  pressure 
and  at  temperatures  up  to  272°  C.,  he  found  the  ionization  to  be 
constant  to  within  about  6 . 5  per  cent  of  the  mean  value.  At  con- 
stant density  he  found  no  greater  variation  in  readings  up  to  201°  C. 
For  hydrogen  at  constant  density  his  results  show  no  variation 
greater  than  about  15  per  cent  up  to  226°  and  for  carbon  dioxide 
no  variation  greater  than  4 . 9  per  cent  up  to  about  the  same  tem- 
perature. Gallarotti  investigated  the  effect  of  temperature  on  the 
ionization  of  air  at  low  temperatures.  With  X-rays  his  results 
show  the  ionization  to  be  constant  to  within  2 . 5  per  cent  of  the 
mean  for  temperatures  down  to  — 187°  C.  With  radium  he  obtained 
measurements  of  the  ionization  at  — 10°,  —60°,  and  — 187°  C.,  which 
did  not  vary  more  than  i .  2  per  cent  from  the  mean. 

These  results  show  that  there  is  no  variation  of  the  phenomena 
with  temperature  such  as  might  have  been  expected  from  the  re- 
sults of  Perrin  on  the  ionization  of  gases.  They  show  that  for  the 
temperatures  considered,  the  rate  at  which  the  electrons  are  sepa- 
rated from  the  atom  does  not  vary  more  than  about  10  per  cent  in 
the  case  of  solids  and  about  5  per  cent  in  the  case  of  gases. 

1  Annales  de  Chimie  el  de  Physique  (7),  n,  496,  1897. 

2  Phil.  Mag.  (6),  7,  81,  1904. 

3  Atti  delta,  R.  Accad.  dei  Lincei,  16,  297,  1907. 

4  Annalen  der  Physik,  19,  333,  1906. 
sProc.  Roy.  Soc.,  A,  82,  351. 


EFFECT  OF  TEMPERATURE  ON  IONIZATION  117 

The  kinetic  theory,  however,  leads  to  the  conclusion  that  the 
variation  of  the  stability  of  the  atom  with  temperature  must  be 
very  slight  and  may  even  be  beyond  the  limits  of  experimental 
determination.  While  the  above  results  are  consistent  with  this 
conclusion,  they  throw  no  light  on  the  question  as  to  whether  a 
smaller  variation  takes  place. 

In  the  present  experiment  an  attempt  has  been  made,  first,  to  re- 
duce the  observational  errors  with  the  purpose  of  measuring  smaller 
variations  than  would  have  been  detected  in  previous  experiments, 
and  second,  to  work  at  higher  temperatures  than  had  been  employed 
in  previous  experiments  on  the  ionization  of  gases. 

The  experiment  was  made  upon  gases  for  the  following  reasons: 
(i)  According  to  the  kinetic  theory  the  molecular  structure  of  a 
gas  is  simpler  than  that  of  solids  and  liquids.  (2)  The  application 
of  the  fundamental  concepts  of  the  kinetic  theory  to  gases  has  been 
more  thoroughly  demonstrated  than  in  the  case  of  solids  or  liquids. 
(3)  The  previous  results  on  gases  are  not  so  accurate  as  some 
of  the  results  on  solids,  and  the  experiments  have  not  been  made  at 
as  high  temperatures  as  should  be  attainable. 

OUTLINE  OF  EXPERIMENT 

The  observations  consisted  in  the  measurement  of  the  ioniza- 
tion current  in  a  gas  within  a  closed  vessel,  by  means  of  the  rate  of 
leak  of  a  charge  to  an  electrometer.  An  attempt  was  made  to 
measure  the  ionization  at  constant  pressure,  but  owing  to  varia- 
tions and  disturbances  due  to  the  change  in  density,  this  method 
was  abandoned  and  all  observations  were  made  with  the  gas  at  con- 
stant density. 

Air  and  hydrogen  were  the  only  gases  studied.  Radium  was 
used  as  the  ionizing  agent.  In  all  the  observations  recorded  here 
the  gamma  rays  were  the  only  radiation  entering  the  ionization 
chamber. 

The  current  was  measured  with  a  quadrant  electrometer  of  the 
Dolezelek  type,  arranged  to  have  a  sensitiveness  of  from  150  to  200 
scale  divisions  per  volt  at  a  distance  of  150  cm.  With  this  sensitive- 
ness the  spot  of  light  from  the  mirror  moved  one  millimeter  in  from 
o.i  to  o.  2  seconds,  a  rate  which  varied  in  different  series  of  obser- 


n8 


/.  HARRY  CLO 


vations.     No  observations  were  made  in  which  the  light  did  not 
move  over  at  least  400  scale  divisions. 

The  temperatures  were  measured  by  means  of  a  gas  manometer, 
whose  minimum  sensitiveness  was  about  one-half  millimeter  per 
degree. 

DESCRIPTION  OF  APPARATUS 

The  apparatus  as  shown  in  the  accompanying  figure  is  as  follows : 
C  is  the  vessel  in  which  the  ionization  took  place  and  whose  tem- 
perature was  varied.  It  was  made  from  an  iron  cylinder  of  about 


FIG.  i 


ii  cm  internal  diameter,  by  reducing  the  walls  to  a  thickness  of  a 
millimeter  or  two  everywhere  except  at  the  ends.  The  side  through 
which  the  rays  passed  was  further  reduced.  Ends  of  heavy  iron 
plates  were  brazed  to  this  wall,  forming  a  cylindrical  chamber  of 
about  1 8  cm  height. 


EFFECT  OF  TEMPERATURE  ON  ION  I Z  AT  ION  119 

The  tube  c  leads  downward  to  the  manometer.  It  is  sur- 
rounded by  a  water-jacket  W.  By  means  of  the  amber  plug  p, 
and  the  cap  which  presses  the  plug  into  position,  it  supports  the 
rod  R  of  about  2  mm  diameter.  This  rod  forms  one  electrode. 
The  amber  plug  gave  satisfactory  insulation  throughout  the  ex- 
periment. 

The  vessel  C  rested  upon  asbestos  and  an  iron  plate.  Surround- 
ing it  was  the  electric  furnace  •  F.  The  furnace  was  surrounded 
by  asbestos  and  the  whole  inclosed  in  an  iron  box  /,  with  walls 
about  2  cm  thick. 

The  manometer  M  was  made  of  a  graduated  capillary  tube. 
It  was  thoroughly  cleaned  and  filled  repeatedly  with  dry  air  before 
using.  For  convenience  in  placing  it  and  in  reducing  the  readings 
to  degrees  of  absolute  temperature,  a  bulb  m  was  blown  in  the 
tube  to  form  a  reservoir  for  mercury,  thereby  keeping  that  arm  of 
the  mercury  at  very  nearly  a  constant  height.  The  whole  air 
column  was  inclosed  in  a  water  jacket  (not  shown  in  the  figure)  to 
regulate  and  determine  its  temperature. 

A  is  an  auxiliary  chamber.  It  consists  of  a  metallic  box  in 
which  a  plate  a  is  held  insulated  from  the  box  by  means  of  an 
amber  plug.  At  the  top  of  the  rod  which  holds  this  plate  is  a 
mercury  cup  which,  with  the  movable  rod  K,  constitutes  the  earth- 
ing key  of  the  system.  This  insulated  system  includes  the  rod  R, 
the  plate  a,  one  pair  of  the  electrometer  quadrants,  and  the  connect- 
ing wires.  U  is  a  layer  of  uranium  oxide  used  to  ionize  the  air  in 
A .  S  is  a  metallic  shield  for  A .  E  is  the  electrometer. 

The  radium  was  held  in  a  lead  block  L,  in  such  a  position  as 
would  expose  all  parts  of  the  gas  chamber  to  the  gamma  rays. 
It  was  of  sufficient  strength  to  give,  in  this  position,  a  measurable 
rate  of  deflection  of  the  electrometer  through  5  cm  of  lead. 

The  electrometer  and  all  connecting  wires  were  surrounded  by 
earthed  conductors  to  prevent  leaks  and  electrostatic  disturbances. 

For  the  source  of  potential  small  storage  cells  were  used.  It  was 
necessary  that  this  potential  be  fairly  constant.  By  letting  the 
freshly  charged  cells  discharge  to  that  potential  which  remained 
constant  for  the  longest  time  and  using  them  in  this  condition,  they 
were  found  to  be  all  that  was  necessary. 


120  /.  HARRY  CLO 

METHOD  OF  OBSERVING,  SOURCES  OF  ERROR 

In  taking  the  observations  the  method  of  procedure  was  as 
follows :  The  vessel  C  was  repeatedly  filled  with  the  gas  by  exhaust- 
ing and  allowing  the  gas  to  flow  in  through  drying  agents.  The 
temperature  of  the  gas  and  the  barometric  reading  were  then  taken. 
The  reading  of  the  manometer  being  taken  at  a  known  barometric 
pressure  and  for  a  known  temperature  of  the  gas  in  the  manometer, 
the  latter  was  sealed  on  to  the  chamber  C.  These  readings  were 
again  taken.  The  values  under  these  known  conditions  gave  the 
constants  of  a  reduction  formula  which  in  turn  gave  the  absolute 
temperature  in  terms  of  the  readings  of  the  manometer  and  the 
temperature  of  the  air  in  the  manometer. 

The  needle  of  the  electrometer  was  charged  to  a  potential  that 
would  give  the  desired  sensitiveness  and  at  the  same  time  minimize 
any  errors  due  to  the  variation  of  this  potential.  The  quartz 
fiber  suspending  the  needle  was  made  conductive  by  coating  with  a 
solution  of  zinc  chloride.  Although  it  was  necessary  to  moisten 
the  fiber  every  week  or  two,  this  method  proved  to  be  more  satis- 
factory than  any  other. 

The  potential  of  the  system  was  held  at  zero,  while  C  was  given 
a  potential  above  or  below  this  value.  This  potential  was  generally 
about  300  volts,  a  value  well  above  that  which  would  give  a  satura- 
tion current.  A  was  held  at  a  potential  of  opposite  sign  to  that  of 
C,  and  of  sufficient  value  to  give  the  saturation  current  caused  by 
the  presence  of  the  uranium  oxide  in  A . 

To  take  a  reading  the  radium  was  removed  from  its  position 
near  C,  the  uranium  oxide  from  its  position  in  A,  and  the  system, 
insulated  by  opening  the  key  K.  Under  ordinary  conditions  there 
would  be  a  perfect  balance  of  any  small  leaks  and  no  deflection 
of  the  electrometer  would  result.  If  a  charge  leaked  into  the 
system  from  C,  the  uranium  oxide  was  inserted  into  A  sufficiently 
to  cause  a  charge  of  opposite  sign  to  pass  into  the  system  from  A . 
By  adjusting  the  position  of  the  oxide  the  system  could  be  kept  at 
zero  potential  indefinitely.  The  radium  was  now  placed  in  position 
and  the  rate  of  the  deflection  of  the  electrometer  needle  was 
measured. 

In  suitable  weather  no  difficulty  was  encountered  with  electro- 


EFFECT  OF  TEMPERATURE  ON  IONIZATION  121 

static  disturbances.  It  was  seldom  necessary  to  use  the  balancing 
device  on  account  of  failure  of  insulation.  The  insulation  was 
always  tested  by  keeping  the  system  at  zero  potential  for  a  period 
much  longer  than  the  time  necessary  for  taking  a  reading.  The 
absence  of  variations  due  to  disturbances  was  considered  sufficiently 
demonstrated  when  successive  readings  at  constant  temperature 
were  found  to  agree  as  closely  as  it  was  possible  to  measure  the  rate 
of  deflection. 

The  first  difficulty  that  was  encountered  was  one  similar  to 
the  disturbance  mentioned  above  as  probably  due  to  change  in 
density.  It  was  found  that  while  the  temperature  of  the  gas  was 
changing,  except  when  that  change  took  place  very  slowly,  the  rate 
of  deflection  of  the  electrometer  was  not  constant  but  varied 
irregularly.  Since  this  variation  was  absent  when  the  temperature 
remained  almost  constant,  it  was  considered  due  to  some  convective 
disturbance  in  the  gas  and  was  eliminated  by  slow  and  careful 
heating  or  by  taking  the  observations  at  constant  temperatures. 

A  second  difficulty  was  found  in  the  expulsion  of  ions  from  the 
hot  metallic  electrodes.  This  could  not  be  overcome  completely 
in  the  apparatus  used  for  these  observations.  It  was,  however, 
partly  overcome.  This  heat  leak1  was  found  to  begin  ordinarily 
at  about  350°  C.,  but,  by  prolonged  or  repeated  heating  at  tempera- 
tures above  this  value,  the  temperature  at  which  the  leak  first 
appeared  was  changed  to  about  450°  C.  Above  this  temperature 
it  was  always  present.  It  was  to  meet  this  difficulty  that  the 
auxiliary  chamber  A  was  introduced  into  the  system. 

To  measure  the  ionization  under  these  conditions  the  tempera- 
ture of  the  chamber  C  was  first  brought  to  as  nearly  a  constant 
value  as  possible.  With  the  radium  removed,  the  position  of  the 
uranium  oxide  was  varied  until  the  system  when  insulated  would 
remain  at  zero  potential  for  a  period  at  least  several  times  as  long  as 
that  required  for  a  reading.  The  radium  was  then  placed  in  posi- 
tion, the  reading  taken,  and  the  balance  again  tested.  If  the 
balance  was  now  disturbed  enough  to  affect  the  reading  by  as 
much  as  o.  i  per  cent  the  reading  was  discarded. 

As  is  well  known,  a  temperature  is  soon  reached  at  which  this 

1  Richardson,  Proc.  Camb.  Phil.  Soc.,  n,  287,  1902. 


122  /.  HARRY  CLO 

leak  increases  very  rapidly  with  the  rise  of  temperature.  At  a 
temperature  of  about  650°  C.,  it  became  impossible  to  keep  the 
temperature  sufficiently  constant  to  be  able  to  balance  the  leak  for 
a  period  great  enough  to  measure  the  ionization. 

At  temperatures  above  600°  C.  the  escape  of  the  gas  from  the 
ionization  chamber  began  to  introduce  another  source  of  error. 
This  difficulty  alone  was  sufficient  to  limit  the  range  of  observations 
to  temperatures  around  this  value. 

DISCUSSION   OF   READINGS,   DATA 

The  results  of  some  of  the  observations  are  shown  in  the  accom- 
panying tables.  The  rate  of  ionization  of  the  gas,  as  represented 
by  the  rate  of  movement  of  the  electrometer  needle,  is  here  expressed 
in  millimeters  per  second.  The  rate  in  any  one  table  is  not  to  be 
compared  with  that  in  another  table,  as  the  sensitiveness  of  the 
instrument  was  not  the  same  for  the  different  series  even  when  the 
same  gas  was  used. 

In  Table  I,  columns  I,  II,  and  III  show  readings  for  air. 
Columns  IV  and  V  are  for  hydrogen.  Columns  I,  II,  and  IV  show 
individual  readings  only.  It  was  impossible  to  work  with  hydrogen 
at  as  high  temperatures  as  those  reached  in  air  because  the  convective 
disturbances  were  much  greatei  than  in  air,  and  because  the  vessel  C 
would  not  hold  the  hydrogen  under  as  great  a  pressure  as  the  air. 

As  one  may  see  from  the  table,  the  individual  readings  for  air 
are  constant  to  within  about  0.25  per  cent  from  the  mean  for 
temperatures  up  to  about  500°  C.  For  hydrogen  column  IV 
shows  about  the  same  uniformity,  but  the  readings  were  taken  to 
about  425°  C.  only.  While  the  individual  readings  of  column 
III  are  not  so  nearly  constant,  the  mean  of  the  readings  at  each 
temperature  shows  a  variation  of  only  about  o .  5  per  cent  from  the 
mean  up  to  about  615°  C.  In  this  series  the  readings  at  the  highest 
temperature  are  subject  to  a  slight  correction  on  account  of  the 
leak  of  air  from  the  vessel.  Either  this  correction  or  the  difficulty 
of  keeping  the  temperature  sufficiently  constant  would  account  for 
the  irregularity  in  the  readings  at  this  temperature. 

In  general  the  readings  agree  as  closely  as  those  taken  under  the 
same  conditions  and  at  the  same  time  upon  the  gas  at  room  tern- 


EFFECT  OF  TEMPERATURE  ON  IONIZATION 


123 


perature.  Hence  the  readings  agree  as  closely  as  the  method  of 
observation  would  warrant,  upon  the  assumption  that  there  should 
be  no  variation  at  all. 

TABLE  I 


Arc 

HYDROGEN 

I 

11 

III 

IV 

V 

Temp. 

Rate 

Temp. 

Rate 

Temp. 

Rate 

Mean 

Temp. 

Rate 

Temp. 

Rate 

Mean 

22°  

25  
6? 

7-93 
7-93 
7.90 
7.92 

7-93 

7  Q3 

20° 

'28 
42 

7-77 
7-77 
7.78 
7-77 
7.78 
7  7% 

21° 

5-66 
5-68 
5.65 
5  68 

5^7 

10° 

6-35 
6-35 

13° 

6-39 
6-39 
6-37 
6.  30 

6.'38 

107 

6.38 

38 

6.39 

6  ^ 

6-37 

77 

7-93 
7.91 
7.90 
7.92 

7-93 
7.90 
7.92 
7.90 
7.92 
7.90 
7.92 
7.92 
7.90 
7.92 
7.91 
7.91 
7-94 

59 
Si 

94 
1  06 
197 
228 
251 
273 
302 

3ii 
323 
332 
359 
398 
424 
441 
466 
497 
515 

7.78 

7-77 
7-77 
7-75 
7-78 
7-75 
7-77 
7.78 
7-75 
7.78 
7-75 
7-79 
7-77 
7-75 
7.78 
7.78 
7-75 
7-75 
7.62 

197  
2OO  

270  
283  
298  
3OQ  . 

396 
442 

5.65 
5-66 
5-66 
5-65 
5-64 
5-64 

116 

6.38 

144 

6-35 
6  38 

.... 

5-66 
5-64 

6-35 
6-37 
6  33 

6.38 

.... 

217 

198 

6.38 

6-37 
6-39 

6-35 

318  
326.  . 

486 
513 

&3 

5-72 
5-67 
5-67 
5-62 
5-69 
5.67 

5-47 
5-8o 

314 

6-35 
6-37 
6-37 
6-37 

6^7 

334  
367  

5-69 
5-66 

366 

6-37 

438  

467  
473  
40  1 

402 

6-37 
6-37 
6  17 

343 

6-37 
6-39 

6-37 

5-63 

429 

6-35 

413 

6-47 
6.27 
6  27 

6^33 

TABLE  II 


Mean  Rate  from 

I' 

ii' 

III' 

IV 

V 

0°-IOO°...... 

7   024 

7    771 

S   667 

6   37"? 

6.380 

IOO  -2OO   
200  -300   
3OO  —400   . 

7-9I5 
7.913 
7    OI4. 

7.765 
7.766 
776^ 

s  6*6 

6.380 

6    17 

6.360 
6.365 

6    37O 

4OO  —500   

7    Q2O 

7    76^ 

*  663 

6  36< 

6    334 

5OO  -6OO   

5.660 

In  columns  I' ',  II',  III',  IV,  and  V  (Table  II),  the  mean  rate 
for  one  range  of  temperature  of  100°  is  compared  with  the  mean  for 


124  J-  HARRY  CLO 

other  equal  ranges.  It  will  be  noticed  that  for  the  higher' tempera- 
tures the  mean  rate  is  in  general  slightly  lower.  This  is  doubtless 
due  to  the  difficulty  in  keeping  the  temperature  sufficiently  con- 
stant or  in  varying  it  slowly  enough  to  avoid  the  disturbances  due 
to  convection  currents  in  the  gas. 

SUMMARY 

The  temperatures  were  varied  from  room  temperature  to  about 
615°  C.  The  absolute  temperature  and  therefore  the  mean  kinetic 
energy  of  the  molecules  was  increased  to  three  times  its  value  at 
room  temperature.  From  the  kinetic  theory  it  may  be  shown  that 
about  i  per  cent  of  the  molecules  have  a  probable  mean  energy 
four  times  this  mean.  Hence  the  energy  of  agitation  of  i  per 
cent  of  the  molecules  was  probably  about  twelve  times  the  mean 
energy  at  room  temperature. 

Readings  were  taken  nearly  300°  C.  above  the  temperature 
at  which  electrons  are  first  driven  from  the  metals  by  heating. 
Both  hydrogen  and  air  were  experimented  with,  the  latter  furnish- 
ing a  desirable  mixture  of  gases  of  different  molecular  weights. 

The  individual  readings  were  in  general  constant  to  within 
o.  2  per  cent  of  the  mean.  In  columns  I',  II',  and  III'  (Table  II), 
which  are  mean  readings  for  air,  the  greatest  variation  is  a  little 
over  o.i  per  cent. 

The  ionization  of  air  by  means  of  the  gamma  rays  from  radium 
is  therefore  independent  of  the  temperature  of  the  gas  to  within  o .  2 
per  cent  up  to  about  600°  C.  For  hydrogen  the  same  independence 
is  shown  for  temperatures  up  to  about  430°  C. 

A  variation  of  over  200  per  cent  in  the  absolute  temperature  of  a 
gas  does  not  affect  the  stability  of  the  atom  sufficiently  to  change 
the  ionization  by  more  than  about  o.  i  per  cent. 

In  conclusion  the  writer  wishes  to  express  his  thanks  for  their 
assistance  and  encouragement  to  Professor  Michelson  and  the  staff 
of  Ryerson  Physical  Laboratory,  and  especially  to  Professor 
Millikan,  under  whose  direct  supervision  this  experiment  was 
undertaken. 

THE  UNIVERSITY  OF  CHICAGO 
January  26,  1911 


JJ3SSS5?""™ 

OVERDUE. 


5>tf    10 


2Jwi 


..  ' 


LD  21-' 


